Is there a difference in using a constructor to create an object versus returning an object?

Question

Is there any difference in how these functions operate? The first one is more typically of what I think about when thinking of a constructor.

Example 1: using t

Answer 1

Basically, when you use new, the JS engine makes a brand new object for you and injects that as the value of this. It also automatically gives you any methods attach to the prototype of the constructor. Using a constructor also allows you to check if an object is an instanceof something more easily.

function MovieA(title) {
    this.title = title;
}
MovieA.prototype.getTitle = function() {
    return this.title;
};

function MovieB(title) {
    return {
    title: title
  };
}
MovieB.prototype.getTitle = function() {
    return this.title;
};

var a = new MovieA('A');
console.log(a instanceof MovieA); // true
console.log(a.getTitle()); // 'A'

var b = MovieB('B');
console.log(b instanceof MovieB); // false
console.log(b.getTitle()); // Uncaught TypeError: b.getTitle is not a function

Everything that new offers you can be attained through other methods but requires more manual labor.

The second method, factories, tend to work better for unit testing, custom object creation and functional programming. It works better for unit testing because if you have a factory producing all of your objects, you can just replace that factory with a mock-up to test different cases.

var Factory = {
  makeThing: function() {
    return { name: 'thing' };
  }
};

// Want to test the case for if makeThing fails inside of some other code
var MockFactory = {
  makeThing: function() {
    return null;
  };
};

As for when you use either, it all depends. Some people don't use new at all. Others exclusively use new. It all depends on if you need any of the things listed above, how much control you need over the creation of objects, when you want to use this or not, etc. In the end, it's all a matter of preference.